In recent decades, research has emerged linking vitamin D to disorders such as schizophrenia and multiple sclerosis. Evelyn Fetterplace and Jana Howden report on vitamin D's moment in the sun.

That feeling of sun on your face in winter not only feels warm and comforting, but the ultraviolet component also helps you produce vitamin D.

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The winter sun streaming down on your upturned face feels fantastic, but it could also be a therapeutic jackpot. Ultraviolet-B rays lets us synthesise vitamin D, and new studies are showing that vitamin D – or lack of – might be behind many chronic conditions.

Before we look at how vitamin D deficiency might change our propensity to various disorders, let’s take a look at how we make the stuff.

We start with a precursor molecule called 7-dehydrocholesterol which is found in human skin (as well as the skin of most other vertebrates). 7-Dehydrocholesterol reacts with ultraviolet wavelengths, between 270 and 300 nanometres, to produce vitamin D3.

We don’t need long to make enough vitamin D each day – depending on the UV index and skin colour, exposing face, arms and legs to sunshine for five to 30 minutes, twice each week, is usually enough. Clothes and sunscreen block ultraviolet-B rays, so this needs to be taken into account, along with a person’s propensity to sunburn.

If you live near the poles, where the long winters are sun-free for months at a time – or just stuck in an office every day – fatty fish, fortified dairy or supplements might be your saving grace.

Vitamin D is essential for growth and development, so it’s no surprise a deficiency can be bad for you. To absorb calcium to build and grow strong healthy bones, for instance, you need vitamin D, and too little vitamin D can result in rickets.

But in the past few decades, research into the effects of vitamin D deficiency has uncovered other potentially long-term effects. Here are just a few examples.

Multiple sclerosis

Our immune and nervous systems both need vitamin D. Crucially, it helps form the protective envelopes around nerves (called the myelin sheath).

With multiple sclerosis, a patient’s immune system attacks the body’s own nervous system. Information flow between nerve cells and other tissues such as muscles goes haywire, causing muscle function, vision and memory losses.

Recently, more evidence has emerged of vitamin D’s influence on multiple sclerosis.

People at risk of the disease that move to sunnier areas seem to lower their chances of it developing. A study published in 2016 of 22,162 multiple sclerosis patients from 22 countries found living closer to the poles is linked to multiple sclerosis symptoms appearing almost two years earlier.

And to prevent its onset, you might need to get in early. A 2016 study on the Finnish population investigated vitamin D levels in early pregnancy. They found that deficiency during pregnancy can double the chance of children developing multiple sclerosis.

Robyn Lucas, an epidemiologist at the Australian National University, says that while there is consistent evidence that lower blood levels of vitamin D are associated with increased risk of multiple sclerosis, there's little evidence that supplements can help those with the condition.

“Vitamin D is not the only chemical being made when skin is exposed to sunlight, and some of these other chemicals may also be important to reducing the risk of multiple sclerosis,” she says.

Studies are underway to test the effects of vitamin D supplementation and non-vitamin D components of sun exposure in people at high risk of developing multiple sclerosis.

Schizophrenia

Vitamin D helps develop the brain – creating connections, and making sure cells know their job. Faults in brain development can lead to neuropsychiatric diseases such as schizophrenia, a severe psychosis disorder.

While work is in its early stages, says John McGrath, a neuroscientist at the University of Queensland in Australia, the importance of vitamin D in the developing brain shows it is biologically plausible that it could have a role in schizophrenia – and there are growing observational studies that support this theory too.

Many studies have shown that people born in winter and spring or at higher latitudes have more chance of developing schizophrenia than summer and autumn babies, or those born near the equator.

A 2010 study conducted in Denmark found low vitamin D levels in pregnant women increased the risk of the children developing schizophrenia.

Another study of Finnish children documented the frequency and dose of vitamin D supplementation in their first year of life. Published in 2004, it showed that vitamin D supplementation in early life drops the risk of schizophrenia for males.

Asthma

Asthma is the most common childhood disorder and sees kids wheezing on playgrounds worldwide – it has become more prevalent in the past half century.

A 2016 study recorded vitamin D concentrations at birth and followed up with children until 10 years of age. It found that vitamin D deficiency in early childhood is associated with increased risk for long-lasting asthma.

How does this work? The authors think vitamin D potentially reduces the chances of developing allergies – a risk factor for asthma – or minimising bacterial infection in a child’s respiratory tract.

And in the case of asthma, vitamin D’s not only a preventative but a treatment too.

A September 2016 review collated data from many studies on asthma and vitamin D to determine whether vitamin D can control asthma symptoms. They found that asthmatics given vitamin D had fewer severe asthma attacks that needed treatment with anti-inflammatory tablets.

Autism-spectrum disorders

Autism-spectrum disorders are a lifelong developmental conditions that affect a person’s ability to communicate with others and interact with the world.

Affecting around 1 in 100 people, people on the autism spectrum can range from having mild symptoms – where a person can live a relatively normal life – to severe ones, where learning difficulties can see individuals relying on continued specialist support for their whole lives.

Despite being highly heritable – meaning that it can be passed from parents to kids – other risk factors have also been linked to autism-spectrum disorders.